U.S. Pat. No. 5,763,483, which is herein incorporated by reference, disclosed carbocyclic compounds and pharmaceutically acceptable salts thereof. Among them Oseltamivir, chemically Ethyl (3R,4R,5S)-4-(acetylamino)-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate is a orally active inhibitor of influenza virus neuraminidase. Oseltamivir is represented by the following structure:

Various processes for preparation of oseltamivir and its pharmaceutically acceptable salts were disclosed, for example, in U.S. Pat. No. 5,763,483, J. Org. Chem., Vol. 63, No. 13, 1998 (page: 4545-4550), J. Amer. Chem. Soc., Vol. 115, No. 4, 1997 (Page: 681-690), U.S. Pat. No. 5,952,375, PCT Publication No. WO 98/07685 and PCT Publication No. WO 99/44185.
PCT Publication No. WO 98/07685 described in example 11 that crystalline oseltamivir free base is obtained by concentrating reaction mass containing oseltamivir free base to obtain oseltamivir free base as a foam, which is solidified on standing. It has been found that the process described in WO 98/07685 does not produce the well-defined crystalline form of oseltamivir free base in a consistently reproducible manner.
Oseltamivir free base obtained by these processes are not satisfactory from purity point of view and there is a need for a process for obtaining consistently reproducible pure crystalline oseltamivir free base.
One object of the present invention is to provide a process for preparation of highly pure crystalline oseltamivir free base that can be used to obtain pharmaceutically acceptable salts of oseltamivir in high purity.
Another object of the present invention is to provide a process for preparation of oseltamivir phosphate in high purity.